In recent years, such systems as computers and communication apparatuses are being downsized and destined to have improved properties, and in this connection, it is desired to improve the technique for forming via holes and through holes (hereinafter simply called through holes) in a ceramic board on which is to be mounted semiconductor devices.
Especially the ceramic green sheet used for CPU, etc. of computer is desired to be higher in accuracy and to be made of a material which allows through holes with a high aspect ratio to be formed.
Multi-layer ceramic boards are generally produced by either the thick film printing lamination method or the green sheet method. The green sheet method can be further classified into the lamination method and the printing method. In the green sheet printing method, a conductive paste and an insulating paste are printed on a green sheet repetitively alternately for lamination, to form a multi-layer board, and after repeated printing and drying, firing is effected only once for completion. The green sheet lamination method is almost similar to the green sheet printing method, but when a multi-layer board is made, each time a conductor is printed is followed by thermocompression bonding and firing. The green sheet lamination method has such disadvantages in that fine through holes cannot be formed at the same time and that many jigs and pieces of equipment must be used. Semiconductor packaging requires techniques for precisely processing ceramic boards, and in the present situation where through holes are formed by mechanical methods such as mold pressing and NC punching and where patterns are formed by screen printing, any new technique to meet the demand for higher precision patterning suitable for smaller circuit materials is required.
As methods to improve these problems, Japanese Patent Laid-Open (Kokai) Nos. 1-183456 and 6-202323 propose the formation of through holes by photolithography using a photosensitive ceramic green sheet. However, since the photosensitive ceramic green sheet is low in sensitivity and resolution, the method has a disadvantage in that through holes with a high aspect ratio, for example, 100 .mu.m or less through holes cannot be formed accurately or uniformly on a thick sheet with a thickness of more than 50 .mu.m.
Furthermore, as a method for forming a circuit wiring pattern on a substrate, screen printing using a paste consisting of a metallic powder and an organic binder has been often used. However, screen printing has a disadvantage in that a highly accurate pattern cannot be formed.
Japanese Patent Laid-Open (Kokai) Nos. 5-204151 and 5-342992 propose methods for forming a pattern by lithography using a photosensitive paste. However, since the photosensitive paste is low in sensitivity and resolution, high accuracy cannot be achieved, and for example, the limit of lines and spaces is 80/80 .mu.m. Japanese Patent Laid-Open (Kokai) No. 4-139733 describes a method for forming a pattern on a film using an ultraviolet light setting resin. In this method, a pasty mixture consisting of a conductive powder mainly composed of one or more high melting point metals selected from W and Mo and a binder mainly composed of an ultraviolet light setting resin is formed as a layer on a film substrate of a flexible resin, etc. using a doctor blade, etc., and the mixture layer is irradiated with ultraviolet light through a photo mask, to harden the binder, for removing the non-irradiated portions. Furthermore, the obtained transfer film is placed on a ceramic green sheet on the conductive pattern side, and the conductive pattern is transferred onto the ceramic green sheet. Finally, the film is removed.
This so-called transfer method allows defects to be found when the pattern has been formed on the film, and can be expected to have an effect of decreasing the failure ratio to improve the yield. However, actual transfer involves many problems, namely how to align the film with a pattern formed on it and the sheet or-substrate on which the pattern is to be transferred, how to maintain the form stability of the film and how to ensure the releasability of the pattern. Furthermore, as in ordinary photolithography, high accuracy cannot be achieved because of the low sensitivity and resolution of the photosensitive paste.